db connction file

<?php

class DB {

    private static $_instance = null; // to use in getInstance function (we can use static keyword)
    private $_pdo, $_query, $_error = false, $_results, $_count = 0;

    private function __construct() {
        try {
            $this->_pdo = new PDO("mysql:host=" . Config::get('mysql/host') .
                    ';dbname=' . Config::get('mysql/db'), Config::get('mysql/username')
                    , Config::get('mysql/password'));
            $this->_pdo->setAttribute(PDO::ATTR_ERRMODE, PDO::ERRMODE_EXCEPTION);
//            print "Connected";
        } catch (PDOException $ex) {
            die($ex->getMessage());
        };
    }

    public static function getInstance() {
        if (!isset(self::$_instance)) {
            self::$_instance = new DB();
        }
        return self::$_instance;
    }

    public function query($sql, $params = []) {
        $this->_error = false;
//        print $sql;
        if ($this->_query = $this->_pdo->prepare($sql)) {
            $x = 1;
//            var_dump($params);
            if (count($params)) {
                foreach ($params as $param) {
//                    print $x;
                    $this->_query->bindValue($x, $param);
                    $x++;
                }
            }
            if ($this->_query->execute()) {
               try{
               if(!$_SESSION['session/session_name']){
                $this->_results = $this->_query->fetchAll(PDO::FETCH_OBJ);
                $this->_count = $this->_query->rowCount();
               }
               } catch (PDOException $pdo){
                   $pdo->getMessage();
               }

            } else {
                $this->_error = true;
            }
        }
        return $this;
    }

    public function action($action, $table, $where = []) {
        if (count($where) === 3) {
            $operators = ['=', '>', '<', '>=', '<='];
            $field = $where[0];
            $operator = $where[1];
            $value = $where[2];
            if (in_array($operator, $operators)) {
                $sql = "{$action} FROM {$table} WHERE  {$field} {$operator} ?";
                if (!$this->query($sql, [$value])->error()) {
                    return $this;
                }
            }
        }
        return false;
    }

    public function get($table, $where) {
        return $this->action("SELECT *", $table, $where);
    }

    public function delete($table, $where) {
        return $this->action("DELETE", $table, $where);
    }

    public function results() {
        return $this->_results;
    }

    public function first() {
        return $this->results()[0];
    }

    public function insert($table, $fields = []) {
        $keys = array_keys($fields);
        $values = '';
        $y = 1;
        foreach ($fields as $field) {
            $values .= '?';
            if ($y < count($fields)) {
                $values .= ',';
            }
            $y++;
        }
     $sql = 'INSERT INTO ' . $table . ' (' . implode(',', $keys) . ') VALUES (' . $values . ')';
//        print $sql;
//var_dump($this->query($sql, $fields));
        if (!$this->query($sql, $fields)->error()) {
            return true;
        }
        return false;
    }

    public function update($table, $id, $fields) {
        $set = '';
        $x = 1;
        foreach ($fields as $name => $value) {
            $set .= $name . ' = ?';
            if ($x < count($fields)) {
                $set .= ',';
            }
            $x++;
        }
        $sql = "UPDATE $table SET $set WHERE id = $id";
        echo $sql;
        if (!$this->query($sql, $fields)->error()) {
            return true;
        }
    }

    public function error() {
        return $this->_error;
    }

    public function count() {
        return $this->_count;
    }

}